1. Field of the Invention
The present invention relates to a method of producing a silicon thin film, in particular, to a method of controlling the thickness of a silicon thin film used in a SOI wafer etc. and of producing a silicon thin film. More particularly, the present invention relates to production of a silicon thin film of any desired thickness by decreasing the thickness of the silicon thin film through wet-cleaning.
2. Related Background Art
There are known the SIMOX (Separation by IMplantation of OXygen) method, the semiconductor bonding method and so forth as a method of producing a SOI wafer.
The SIMOX method uses a technique for forming a silicon oxide layer in a silicon substrate by implanting oxygen ions into the silicon substrate from its surface and then annealing the resultant substrate at a high temperature to form the silicon oxide layer at the portion where the oxygen ions have been implanted. In this method, the energy for implanting oxygen ions and the dose of the same cannot be set arbitrarily, but they are fixed to approximately constant conditions. Accordingly, it is difficult to set arbitrarily the thickness of the silicon film or that of the oxide film formed by ion implantation in the production of a SOI wafer.
On the other hand, there are several types of semiconductor bonding method. The first one is referred to as xe2x80x9cbonding and polishing SOIxe2x80x9d method.
In the xe2x80x9cbonding and polishing SOIxe2x80x9d method, two silicon wafers at least one of which has been oxidized, are previously prepared, bonded to each other at room temperature and annealed, and then polished from its one side, to leave a silicon film of a desired thickness on the silicon oxide layer. According to this method, both the thickness of the silicon layer and that of the implanted oxide layer can be set arbitrarily. In this method, however, as means of making a thin film of a silicon layer are solely used grinding and polishing. Accordingly, it is difficult to obtain a thin film of a uniform several hundred nm thickness because the limitations of the original thickness accuracy and polishing accuracy in the silicon wafer.
As the measures to overcome the above difficulty, a technique for forming an ultrathin film (100 nm thick or less) uniformly in which thickness distribution of a SOI film is measured instantly and dry-etching is performed relatively heavily at regions where the film is rather thick while dry-etching is performed relatively lightly at regions where the film is rather thin has been reported. This technique is referred to as PACE (Plasma Assisted Chemical Etching). The PACE system includes two units separated from each other: a unit for measuring instantly thickness of a SOI film at in-plane multiple points (10000 points or more) and a unit for performing plasma etching. The unit for performing etching has a plasma generating portion in the form of a nozzle and is designed in such a manner that the nozzle can move over a wafer along its surface and generate plasma according to the measurements of the thickness of the SOI layer so as to etch, for example, the rather thick regions relatively heavily. This technique allows the control of the etching amount from region to region within the wafer surface and hence the control of the absolute value and uniformity of the film thickness. However, the surface having been subjected to plasma-assisted etching has etching damage remaining thereon; accordingly, in many cases, the surface of the SOI layer is subjected to polishing so as to remove its damaged layer. This polishing operation may cause non-uniformity again in the film thickness of the SOI layer which has just been made uniform.
In another type of semiconductor bonding method, as disclosed in U.S. Pat. No. 5,374,564, a SOI structure is produced in the following three steps: implanting hydrogen ions in a silicon wafer having an oxide film formed thereon from its surface to form a brittle layer inside the wafer, bonding the wafer to another wafer, and heat-treating this bonded wafer or spraying a fluid (a liquid such as water or a gas such as nitrogen) on the side of this bonded wafer to separate the brittle layer.
In this method, the film thickness can be controlled by the thickness of the oxide film of the wafer prepared at the beginning and the energy for implanting hydrogen ions. However, in many cases, polishing is required as a finishing operation to the roughness of the separated SOI surface, which may cause again non-uniformity in the film thickness of SOI layer.
In still another type of semiconductor bonding method, as disclosed in U.S. Pat. No. 5,371,037 (Japanese Patent No. 2,608,351), Japanese Patent Application Laid-Open No. 5-21338 or Japanese Patent Application Laid-Open No. 7-302889, a SOI layer is produced in the following two steps: bonding an epitaxial silicon single crystal film grown on a substrate having a porous silicon thereon to another wafer via an oxide film and removing unnecessary portions. In this method, the thickness of the SOI layer is controlled through controlling the thickness of the epitaxial film and that of the oxide film.
In the step of selectively etching the porous silicon remaining on the surface of the SOI layer, the surface may become rough, as seen from the observation after completing the step; however, the rough surface can be changed into a very smooth surface by hydrogen annealing the surface of the SOI layer, as disclosed in Japanese Patent Application Laid-Open No. 5-218053. This method does not permit the deterioration of thickness distribution of the SOI layer to occur.
However, even in this method, it is not easy to form an ultrathin film with thickness 100 nm or smaller.
In the PACE method, the layer damaged by the plasma assisted etching and remaining on the surface of a SOI layer is removed; therefore the SOI layer must be formed to a little larger thickness allowing a little thickness for removing. However, the damaged layer is removed by polishing, thereby variations occur in film thickness distribution, which makes it difficult to form a uniform ultrathin film. For the same reason, in every and each method in which a SOI layer requires polishing, it is difficult to directly form an ultrathin film.
In the cases where the hydrogen annealing described above is adopted to smooth a SOI film, pinholes may be created. Microregions on the semiconductor bonding interface side of an ultrathin film can be stressed due to dust particles, which are too fine to measure with measuring instruments (90 nm or smaller), existing at the semiconductor bonding interface and due to surface irregularities of the wafer itself. If hydrogen annealing is performed in the presence of this stress, pinholes can be created at the portions the stress is established.
Meanwhile, when thickness of a SOI film required in designing a semiconductor device, such as transistor, is much thinner than the least possible thickness which can be supplied as a SOI wafer, or when SOI wafers of different thickness are required with change in design, supply of wafers cannot fully keep up with the demands.
In such a case, in order to obtain a SOI layer having a desired film thickness, manufacturers of semiconductor devices have to take the steps of: getting previously a SOI wafer having a SOI layer thicker than the designed one and subjecting the SOI layer to sacrificial oxidation in which the surface of the SOI layer is once subjected to thermal oxidation and then the oxidized portions are removed by etching.
However, performing sacrificial oxidation causes speed increasing oxidation the crystal defects existing in the SOI film and inhibits oxidation in the vicinity of the regions on which dust particles are deposited; as a result, surface roughness is caused on the surface of the SOI layer. This in turn causes deterioration of high pressure resistance of the oxide film in semiconductor devices.
Accordingly, the object of the present invention is to provide a method for controlling the thickness of a silicon thin film, a method of producing a silicon thin film and a method of constructing a SOI substrate which enable the thickness of a silicon thin film to be decreased to a desired value without deterioration of the quality of the silicon thin film.
The present invention is a method for producing a silicon thin film provided on an insulating surface, the method being characterized by comprising a step of wet-cleaning the silicon thin film on the insulating surface to decrease the thickness of the silicon thin film to 100 nm or smaller.
Further, the method for producing a silicon thin film is characterized in that a first thickness of the silicon thin film, which means the thickness before it is subjected to the wet-cleaning, is larger than 100 nm and the wet-cleaning is performed after the silicon thin film with the first thickness is heat-treated in the reducing atmosphere containing hydrogen and until the thickness of the silicon thin film is decreased to a second thickness smaller than the first one, the second thickness being 100 nm or smaller or 50 nm or smaller.
The method for fabricating a SOI substrate according to the present invention is characterized by comprising the steps of: preparing a substrate having a silicon thin film with a first thickness larger than 100 nm on an insulating surface; heat-treating the substrate in the reducing atmosphere containing hydrogen, and wet-cleaning the heat-treated substrate to decrease the thickness of the silicon thin film to a second thickness smaller than the first thickness, the substrate being prepared in the following steps of: forming a composite member by bonding a first substrate, which includes a silicon thin film formed on a porous layer, to a second substrate in a manner that an insulating layer exists between the first substrate and the second one and dividing the composite member at the porous layer, or the substrate being prepared in the following steps of: forming a composite member by bonding a first substrate, which includes an ion implanted layer and a silicon thin film formed thereon, to a second substrate via an insulating layer and dividing the composite member at the ion implanted layer.
According to the present invention, it is possible to avoid the surface roughness due to the speed increasing oxidation of crystal defect portions occurring when conducting the conventional sacrificial oxidation, the effect of the dust particles, etc. and also avoid deterioration of the high pressure resistance of the oxide film which is attendant on the surface roughness. In addition, according to the present invention, it is possible to omit the steps of thermal oxidation and of etching the oxide film formed by thermal oxidation.